1. Technical Field
The present invention relates generally to cellular communication systems and in particular to a fixed wireless communication system. Still more particular, the present invention relates to a method and system for more effective power control in a fixed wireless communication system.
2. Description of the Related Art
Fixed wireless cellular communication networks are often established in a geographical location in which a public switched telephone network (i.e., landline) is not available. In a fixed wireless cellular communication network, a base station communicates with a plurality of fixed wireless stations that are often used in place of routine public switched telephones. Because the fixed wireless stations (fixed wireless terminals) serve as a substitute for ordinary landline phones, their physical characteristics and the manner in which they are used are different from ordinary cellular mobile stations (eg., cellular phones).
For example, the fixed wireless terminals are typically placed in one location for a period of time without being moved to another location. One reason for this is that a typical fixed wireless terminal is much larger and heavier than an ordinary palm sized mobile station. The fixed wireless terminals are larger because they have to transmit at higher power levels to be able to communicate with a remote base station. In a typical fixed wireless system, the fixed wireless cells, or geographic areas served by one base station, are often larger than routine cellular cells, and thus, transmission power requirements are higher. The higher transmission power requirements in turn lead to the use of power transmitters that are both larger and heavier.
During their operation, mobile stations encounter spatial (or spaced based) fading (shadow fading) which is primarily caused by the introduction and removal of obstructions (and multi-path routes) between the base station and the mobile station. The shadow fading changes rapidly over time as the mobile station moves during its operation.
Because fixed wireless terminals frequently remain stationary during their operation as compared with ordinary mobile stations, common types of interference are often observed for longer periods. The fixed wireless terminal is unable to change its position in relation to the base station and to the source of the interference. Fading encountered by fixed wireless units is temporal (time-based) and is primarily caused by the moving environment (e.g., scattered rain showers). If a given signal received by a fixed wireless unit experiences a fade, the fade will frequently last for a sufficiently long period to cause the fixed wireless terminal to receive undesirably poor signal quality due to undesirably high frame error rates.
Typically, mobile systems operate according to the IS-95 standard. The IS-95 defines procedures for a complex power control method, designed to save on battery life and help prevent co-channel interference. The IS-95 standard has a basic algorithm that is implemented in both fixed and mobile networks. However, the IS-95 power control algorithm is designed for a mobile network. For a fixed wireless environment, the algorithm does not perform well, as described below.
The IS-95 standard is utilized within the Code Division Multiple Access (CDMA) protocol which is in turn used in cellular networks. In a CDMA-based cellular system, the system capacity (i.e., the number of simultaneous users supported by the system) is the most important factor affecting the revenue of the service provider. Higher capacity is desirable since it leads to higher revenues. The system capacity is usually limited by the forward link power available at the base station. Fixed wireless systems experience long fades which result in higher power requirements due to the characteristics of the propagation environment.
Power control is therefore one of the critical operations in CDMA cellular systems. Power control is performed on both forward and reverse links. Efficient power control can help achieve high system capacity. The capacity of a CDMA system, N, is inversely proportional to Ptraffic (which is the traffic control channel power). A good power control algorithm can reduce Ptraffic, thereby increasing the capacity (N) A good power control algorithm also maintains a reasonable frame error rate (FER).
As described above, in a fixed wireless environment, temporal fades (due to unresolvable multipaths) are relatively long (i.e. spanning over several frames), causing several consecutive frame errors. These errors degrade voice quality and result in several gain increases (i.e., increases in the power requirements) to maintain a given FER. This in turn leads to a decrease in the system capacity. In a CDMA system, the system capacity is very sensitive to the power requirements. A good power control algorithm reduces the power requirements, increasing the system capacity. Thus, the existing power control algorithms are inefficient from the power conservation perspective.
In a fixed environment, fades last long, causing several frame errors. These errors cause the power control algorithm to demand more power for the user to maintain a certain quality of service. An existing algorithm is designed for short fades (observed in mobile networks) and does not perform well in a fixed environment. In the existing algorithm, there is one large step up in power and 100 small steps down in power for one frame error. Such power control loop operation helps achieve 1% FER.
The present invention thus recognizes that it would be desirable to have a method and system for effectively controlling power in a fixed wireless system. It is further desirable to have a power control method, which prevents unnecessary power increases due to long temporal fades, while maintaining a reasonable frame error rate (FER). It is further desirable to increase a capacity of a fixed wireless system by controlling the power usage during temporal fades.
It is therefore one objective of the present invention to provide an improved cellular communications system.
It is another objective of the present invention to provide an improved fixed cellular communications system.
It is yet another objective of the present invention to provide an improved fixed cellular communications system utilizing a method and system for controlling power, which increases system capacity by reducing the effects of temporal fades on the system""s power.
The foregoing objectives are achieved as is now described. A method for efficient power control in a (CDMA based) fixed wireless system is disclosed. The method comprises the steps of (1) monitoring a transmission link of the fixed wireless system for an occurrence of a fade, (2) recording a number of power increases to the transmission link due to said fade, and (3) reducing the power to an appropriate level when said fade is completed. The adjusting step includes the steps of determining an appropriate power-down step size, which results in saving power, wherein the step size is influenced by a frame error rate (FER) and selected to maintain a desired FER, and dynamically decreasing the power by the appropriate power-down step size, thereby conserving power and increasing system capacity.
In a preferred embodiment of the invention, the method is implemented with an algorithm which causes the opening of an observation window upon receipt of a PMRM. The time instances of gain increases are recorded, and then the observation window is closed when consecutive good frames occur. A preselected threshold, T, corresponding to a certain number of frames, is used as a parameter in the algorithm. If the first good frame of the consecutive good frames occurred within T frames of a gain increase, the digital gain is reduced by an amount equal to all subsequent gain increases after the gain increase, thus implementing a fast power-down operation.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.